The Internet is a worldwide network of interconnected computers and computer networks. The Internet includes Internet service providers (ISPs) who provide Internet access to a large number of subscribers or users who pay the ISP a fee for the access. The subscribers can be individuals or organizations and can access the Internet by connecting to the ISP using modems over the general switched telephony system (GSTN), or cable television networks, or other types of communication line infrastructure. The Internet also includes vendors who advertise products and services over the Internet and solicit orders from users. Commercial transactions over the Internet can be performed in a variety of ways. A preferred way of payment is by credit card. However, because of security reasons there is great reluctance of the users to transmit credit card account information over the Internet. Additionally, in situations were the transaction amount is small, for example, a few dollars or less, it may not be practical for a vendor to use a credit card transaction. PCT international publication No. WO 97/03410 to Egendorf discloses a method for Internet billing. According to which an ISP can bill a billing acount of a customer for products or services purchased from a vendor over the Internet, avoiding the need to communicate the customer's credit card or account number over the Internet. However, the method is limited in that each ISP has to establish agreements with a multiplicity of vendors some of which may not be subscribers of the ISP. Furthermore, each vendor has to establish separate agreements with a multiplicity of ISPs to be able to use the method of WO 97/03410 for performing transactions with customers of different ISPs.